What happens to plants in winter?

As winter approaches, deciduous trees transfer nutrients from the leaves into the branches where they're deposited in the bark (Source: frankiefotografie/iStockPhoto)

Even though Australia doesn't experience the long, harsh winters of the northern hemisphere, your lawn might have stopped growing, leaves are falling off deciduous trees and some seeds are weeks or months away from germinating.

This period of dormancy, where plants are ticking over but not growing, comes with the falling temperatures and reduced day length that winter brings.

But this dormancy is much more than a period of suspending animation. It's part survival mechanism, part housekeeping exercise, all meant to help plants gear up for warmer days ahead.

Let's start with seeds in the ground, perhaps tucked away in a corner of your vegie patch.

If the seeds are from an introduced species that originated from a colder climate, they will be genetically programmed to survive winter, says Dr Mark Ooi, a plant ecologist who studies seed dormancy at the University of Wollongong.

"Garden species from the northern hemisphere evolved to survive these harsh conditions. They're still adapted to the conditions in which they evolved," he says. They just haven't been in Australia long enough to adapt to local conditions.

Ooi says it's only when a seed has been exposed to low temperatures for long enough, a process known as cold stratification, that plant hormones trigger the end of dormancy. At this time, if environmental conditions are favourable — say there's enough water around — the seed can then germinate, again a process governed by plant hormones.

Seasonal patterns not only affect seeds but the whole plant, says Associate Professor Brian Atwell from Macquarie University, who studies how plants grow in harsh environments.

He says that a drop in temperature slows down a plant's metabolism largely because the enzymes that drive these biochemical reactions don't work so well in the cold. Photosynthesis slows, respiration slows, growth stops.

A classic example is your backyard lawn, Atwell says, which stops growing over winter if the temperature is low for long enough.

Being exposed to less sunlight also plays a role in grass growth this time of year due to the lower angle of the winter sun and shorter winter days, he says. That means less photosynthesis, which in turn means fewer sugars to metabolise.

"If you can't make the energy, you might as well close up shop," says Atwell. "You're not making enough for growth."

The result? Gardeners don't have to mow the lawn for a while.

Many native Australian plants also have a slower metabolism in winter, slowing growth, he says. But most do not undergo a period of physiological winter dormancy, like many introduced species.

Atwell also says that plants use winter dormancy to keep their houses in order. For instance, proteins are broken down and re-made and cell membranes are maintained.

Winter dormancy can also be about conserving nutrients. That's one of the reasons, says Atwell, why deciduous trees shed their leaves at this time of year.

As winter approaches, leaves lose chlorophyll and the tree salvages its constituents — mainly nitrogen, magnesium and phosphates — for recycling. The nutrients are carried back from the leaves into the branches where they're deposited in the bark.

Plant hormones (first auxin, then ethylene) then trigger the leaves, which are now largely stripped of nutrients, to fall off the tree.

Buds can also lie dormant over winter, often covered in scales, until the plant been exposed to low temperatures for long enough. Cherry trees, for instance, are genetically programmed to undergo a winter before buds open in spring. So, even if we have an unusually warm winter, buds won't burst to life until the tree has been chilled.

But how do plants 'know' or register when they've been exposed to enough cold weather?

"Plants have a temperature memory," says Atwell. "They measure the product of time and temperature and can work out how cold it's been and for how long."

They do this, he says, by keeping track of the interactions between certain proteins, a sign that it's time to activate a key gene to break dormancy.

Some people say this sequence of events is evidence that plants do maths to calculate the product of time and temperature. Others dispute whether plants are computing anything at all.

Either way, there's clearly more going on in your winter garden than you might think — maths or not.

Breaking the rulesWhile some native Australian seeds from cold-climate regions also lie dormant in winter, until recently people had assumed that other native seeds didn't take seasonal patterns into account when 'deciding' when to be dormant and when to break dormancy.

But what Dr Mark Ooi from the University of Wollongong and his team have discovered is that some native seeds, for example boronia seeds, show seasonal patterns of dormancy as well as needing fire to jump-start germination. Fire alone isn't enough.

Other special cases are desert seeds that evolved to be dormant during the harshness of a dry season (they undergo warm stratification) and seeds originating from a Mediterranean climate that evolved to stay dormant until rains come at the start of winter, says Ooi.

In both cases, seeds evolved to break dormancy under conditions that maximise the survival of the fragile seedling into a mature adult plant.

Another anomaly is the magnolia tree, says Associate Professor Brian Atwell from Macquarie University, which he says is "genetically fixed" to flower in winter rather than in spring. This, he says, is a legacy of its Chinese ancestry, where local conditions would have favoured this pattern of dormancy and budding.

About the authorAnna Evangeli writes about the science of gardening at her award winning blog The Geeky Gardener. She is a former news editor at ABC Science and science correspondent for ABC Gardening Australia magazine, and has degrees in biochemistry and journalism.